In data communications using discrete mulitone (DMT) technology, a serial data bit stream to be communicated is distributed among multiple channels and transmitted in parallel from a transmitting modem to a receiving modem. These channels are contained in bandwidth of approximately 1.104 megahertz. Each channel has a bandwidth of 4 kilohertz. The center frequencies of each band are separated by 4.3125 kilohertz.
It is often the case that interference with each channel may vary depending on the precise frequency band occupied by the particular channel. Some channels may experience little or no interference, while the interference on others may be so great as to make that particular channel unusable.
Because interference with individual channels may vary, conventional DMT modems establish a bit loading configuration in which the bit rate of individual channels varies based upon the signal-to-noise ratio of each channel. In order to determine the individual bit rates for each channel, the signal-to-noise ratio for each channel is ascertained. This is typically accomplished at the start up of data communication by transmitting a tone at the center frequency of each channel from the transmitting modem and then measuring the signal-to-noise ratio for each channel at the receiving modem.
After the signal-to-noise ratio is measured for each channel, it is a typical practice to subtract a common margin, typically 6 dB, from the measured signal-to-noise ratio of each channel to obtain a transmission signal-to-noise ratio at which to achieve a bit error rate of approximately 10.sup.-7. An appropriate bit rate is assigned to the channel based upon the transmission signal-to-noise ratio obtained.
Such conventional approaches suffer from the subtraction of a common margin from all of the measured signal-to-noise ratios of each channel. It is automatically assumed that the common 6 dB margin is appropriate to compensate for the variation in the signal-to-noise ratio for each channel. However, some channels may experience greater variation in the signal-to-noise ratio than others. Thus, in some cases the common margin may be too great, resulting in a bit rate that is unnecessarily slow. In other cases the common margin may be too small, resulting in a bit rate that is too high which translates into an unnecessarily high bit error rate.